Changing Firefox profile location

1. Prepare a local directory
mkdir -p /data/`/bin/hostname -s`/$USER/.mozilla/firefox

2. Copy the profile directory to the new, local, location. The profile directory name will consist, usually, of alphanumerics, followed by .default. In this example it is called qbzybi74.default.
cd ~/.mozilla/firefox
cp -rp qbzybi74.default /data/`/bin/hostname -s`/$USER/.mozilla/firefox/.


3. Edit ~/.mozilla/firefox/profiles.ini to reflect the new location. 
Change the line
Path=qbzybi74.default
to
Path=/data/HOST/USER/.mozilla/firefox/qbzybi74.default
replacing
HOST by the short name of your computer (eg parrot)
USER by your username (eg flint)

4. Check Firefox starts and the profile has been preserved.
NB this will mean  that firefox will only provide a full profile on the local system but as Firefox is generally only run from a local graphical session this should not matter.

If you move system you will need to 
- Transfer the data from /data/HOST/USER/.mozilla/firefox to a local directory on the new computer
- Edit ~/.mozilla/firefox/profiles.ini to reflect the new location. The HOST entry in the line beginning Path= will need to be changed.